1. Field of the Invention
The present invention relates to an EL (electro luminescent) element drive circuit and an organic EL display device using the same drive circuit. In particular, the present invention relates to a circuit layout of a current drive circuit for driving organic EL elements having anodes connected to a column line of an organic EL panel of an organic EL display device through terminal pins of the organic EL panel by currents generated by a D/A converter circuit constituted with a current mirror circuit, with which luminous variation of the organic EL elements can be reduced to reduce influence thereof on luminous variation of every organic EL display device.
2. Description of the Prior Art
An organic EL display panel of an organic EL display device, which is mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance) and includes 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines, has been proposed and the number of column lines and the number of row lines of such organic EL display panel tend to be further increased.
A current drive circuit of such organic EL display panel includes an input stage and an output stage. The output stage of the current drive circuit includes an output circuit constructed with a plurality of drive circuits, which are, for example, current-mirror circuits and are provided correspondingly to respective terminal pins of the organic EL display panel, regardless of the type of drive current, the passive matrix type or the active matrix type.
The output circuit constructed with the current mirror circuits, each of which is provided for each terminal pin and a drive stage of the output circuit includes parallel drive current mirror circuits constituting a reference current distributor circuit, which includes a plurality of output side transistors provided correspondingly to terminal pins of the organic EL panel, as disclosed in, for example, U.S. Pat. No. 6,586,888 corresponding to JP2003-255898A, which is hereby incorporated by reference. The current mirror circuits of the output circuit are responsive to a reference current supplied from a reference current generator circuit as an input stage of the current mirror circuit to generate mirror currents corresponding to the terminal pins to thereby drive the output circuit by distributing the mirror currents to the respective terminal pins. Further, D/A converter circuits having a current mirror construction are provided on an upstream side of the output circuit correspondingly to the respective terminal pins to D-A convert the mirror currents to thereby generate analog currents according to display data for the terminal pins. The analog currents thus generated drive the output circuit having the current mirror circuit construction to generate drive currents for the organic EL elements provided correspondingly to the terminal pins.
By generating the drive currents for the respective organic EL elements with using the current mirror circuit, it is possible to amplify the reference current, which is as small as several micro amperes in the passive matrix type, to about 1A with restricted power loss. Therefore, a drive circuit having low power consumption can be realized. In the active matrix type, it is possible to obtain the drive current in the order from 1 nA to 10 μA with low power consumption, similarly.
In such current drive circuit of the organic EL display panel, a number of unit transistors constituting the current mirror circuits are arranged for pads provided correspondingly to the respective terminal pins.
FIG. 4(a) to FIG. 4(c) show a conventional layout of unit transistors forming D/A converter circuits as output stage current sources of a current drive circuit and a construction of the unit transistor. In detail, FIG. 4(a) shows a layout of unit transistors, in which transistor blocks 3 each including two transistor columns each including a plurality of unit transistors 1 arranged perpendicularly to an arranging direction of pads 2 connected to respective output terminals of the organic EL display panel. A plurality of the transistor blocks 3 are arranged correspondingly to the respective pads 2. The current drive circuit provided as an IC for each output terminal of the organic EL display panel is constructed with a plurality of the transistor blocks 3.
As shown FIG. 4(c), which is a cross section of the unit transistor 1, the unit transistors 1 are isolated from each other in a rectangular region formed by a buried layer (B/L) 11 and element isolating region 12 and a base region 13b, an emitter region 13e and a collector region 13c of the unit transistor 1 are formed on a surface side of a substrate 16. Incidentally, reference numerals 14b, 14e and 14c shown in FIG. 4(b) and FIG. 4(c) depict a base contact, an emitter contact and a collector contact, respectively. The collector contact 14(c) is connected to the collector region 13c through a collector wall 15 and the buried layer 11.
A reference numeral 17 depicts an interlayer insulating film formed of NSG and a reference numeral 18 depicts an insulating layer formed of polysilicon (P-SiN) formed on the interlayer insulating film.
A pair of the transistor blocks 3, each of which includes a plurality of unit transistors 1 arranged perpendicularly to a pad arranging direction as a column, are provided for each pad as shown in FIG. 4(a). The current drive circuits provided correspondingly to the terminal pins of the organic EL display panel are constructed as an IC by integrating a plurality of paired transistor blocks 3 provided successively in the arranging direction of pads, which are provided correspondingly to output terminals of the current drive circuit.
Since one of the paired transistor blocks 3 corresponds to one of the R, G and B display colors, 3 pairs of the transistor blocks 3 are used as one set and a plurality of sets are provided for the pads successively.
When a pad pitch is, for example, 50 μm, and a width of an element forming area of the transistor block 3 in the pad arranging direction is 44 μm, 2 unit transistors are formed in parallel in the element forming area in the pad arranging direction with a gap of 6 μm between adjacent transistor blocks and an enough number of the transistor blocks 3 for constituting the drive circuits for the respective terminal pins in a direction perpendicular to the pad arranging direction.
As a result, the two unit transistor columns each including, for example, several tens to one hundred unit transistors 1, length of which is 1.0 mm to 3.0 mm, are arranged in the perpendicular direction correspondingly in position to each of the pads 2 to be connected to the output terminals. Almost all of these unit transistors 1 are current-mirror connected in a wiring layer above the insulating layer 18.
The current mirror output circuits constituting the output stage current source and the current switching D/A converter circuits constructed with current mirror circuits are constructed with paired transistors, which have substantially identical paring characteristics and selected from the paired unit transistors in the unit transistor columns.
In this description, the term “current switching D/A converter circuit” means a circuit for obtaining analog currents by ON/OFF switching currents of a plurality of current sources according to bit data.
The 2 columns of unit transistors 1 provided for each pad 2 are shown in FIG. 4(a). However, in order to improve the integrating efficiency, 6 columns of unit transistors for 3 terminal pins for R, G and B display colors may be used as one set corresponding to a pixel pitch of R, G and B display colors and a plurality of the sets are provided successively in row direction. In such case, although the width of the 6 columns of the unit transistors in the pad arranging direction becomes 150 μm, the wiring of the unit transistors is identical to that in the case of the 2 column set.
A drive circuit for driving organic El elements by current-driving organic EL elements arranged in a matrix and resetting the organic EL elements by grounding anodes and cathodes of the organic EL elements is disclosed in JPH9-232074A. Further, a technique for current driving organic EL elements with reduced power consumption by using a DC-DC converter is disclosed in JP2001-143867A.
In a case where large drive currents in the order of milli-ampere or ampere are generated on the basis of a reference current in the order of several micro amperes by the organic EL element drive circuit in which the drive currents are generated by using the D/A converter circuit having the current mirror circuit construction mentioned previously, there is a problem that variation of conversion accuracy of the D/A converter circuit causes variation of output current of an output stage current source, resulting in a luminous variation.
Particularly, in the current switching D/A converter having a current mirror construction, since the currents, which are, for example, twice, four times, eight times, sixteen times the reference current or more, obtained at the output side transistors of the current mirror circuit by switching the current from the current source, the number of the transistors constituting the current mirror circuit becomes several tens to one hundreds. Therefore, it is important to maintain the paring characteristics of unit transistors of the current mirror circuit with respect to the input side transistors thereof. In the IC chip, the pairing characteristics is influenced by distance between paired transistors and positions in the transistor block forming area of the IC, from which the paired transistors are selected.
Further, in order to generate the drive currents corresponding to the display data indicative of luminous intensities of the organic EL elements, the D/A converter circuit constructed with the current mirror circuit and having high conversion accuracy is required. When the vertically long unit transistor blocks 3 are arranged correspondingly to the pads, the pairing characteristics of unit transistors at opposite side positions of the transistor block forming area are degraded.
The reason for this is that, although a transistor block 3 positioned inside the transistor block forming area has other transistor blocks 3, each of which is similar to the inside transistor block 3 on both sides thereof, the transistor block provided at the end of the transistor block forming area has another transistor block on one side thereof. Therefore, the environments in physical properties of the transistor blocks 3 formed in the opposite side portions of the transistor block forming area are not equal or similar to those of the transistor blocks 3 formed inside the transistor block forming area so that there is a difference in characteristics between the transistor at either end portion of the transistor block forming are and the transistor formed inside the area. Therefore, the pairing characteristics of these transistors forming the current mirror circuit is degraded.
As a result, the difference in output current between terminal pins corresponding to the current mirror circuits becomes large, resulting in variation of luminous intensity of the organic EL elements. Particularly, when a plurality of drive ICs are used, the luminous variation at boarder of the ICs becomes conspicuous.
In U.S. Pat. No. 6,586,888, the luminous variation in the boarder between ICs is prevented by arranging an input side transistor of a current mirror circuit for distributing a reference current in a center portion of a plurality (several tens) of output side transistors of the same current mirror circuit.
FIG. 5 shows output current characteristics of the current mirror circuit disclosed in U.S. Pat. No. 6,586,888. That is, FIG. 5 is a graph showing the inter-terminal output current characteristics of the current drive circuit with 45 terminal pins for each of R, G and B display colors. In FIG. 5, the ordinate axis indicates current variation rate (%) and the axis of abscissas indicates the terminal pin numbers corresponding to positions of the terminal pins. A thin sequential line shows measured inter-terminal output current characteristics and a thick line shows a regression curve.
As is clear from FIG. 5, the closer the terminal pin to either end portion of the transistor block forming area is, the smaller the output current is. The terminal pins at the opposite ends of the transistor block forming area correspond to the transistor groups 3 at the opposite end portions of the transistor group forming area in a substrate of the IC, respectively.
Even if the reference current is distributed by arranging the input side transistor of the current mirror circuit in the center portion of the output side transistors as disclosed in U.S. Pat. No. 6,586,888, it is impossible to sufficiently restrict the luminous variation on the display screen of the organic EL display panel. The luminous variation of every organic EL display device is strongly influenced by the reduction of output current of the transistors in the opposite end portions of the transistor block forming area.